Although the invention is not limited to such applications, it will be more particularly described with reference to layers used to make the light emitted from a light box, in particular a flat lamp homogeneous. Such a flat lamp may especially be a back-light source, used especially in flat-screen computers to illuminate a liquid-crystal screen. They may also be architectural flat lamps used, for example, on ceilings, floors or walls. They may also be flat lamps for municipal applications, such as lamps for advertising panels or lamps able to constitute the shelves or backs of display cabinets.
These flat lamps may also find applications in other fields, such as, for example, the automobile industry since it is conceivable to produce motor-vehicle roofs part of which includes such a lamp, particularly to substitute for the currently known illumination of the passenger compartment of a motor vehicle. It is also possible to produce the backlighting for motor-vehicle dashboards.
Moreover, the expression “flat lamp” should be understood as corresponding to a construction made from two substrates which are initially substantially flat but which, however, may have a slight curvature for a given application.
These flat lamps, as described for example in the U.S. Pat. No. 6,034,470, therefore consist of two substantially flat substrates, such as glass sheets, on which various layers making up the lamp are deposited. For example, on the first glass sheet, which is the rear sheet of the lamp, silver electrodes coated with a dielectric are deposited on the internal face and layers of alumina and phosphor are deposited on the other face. Layers of alumina and phosphor are deposited on the internal face of the other glass sheet, the alumina layer forming reflection regions allowing the light emitted by the said lamp to be made homogeneous. Other materials, such as titanium oxide, may also serve as reflecting layers.
However, it is apparent that the light thus emitted, especially in the case of backlighting for liquid-crystal screens, is not sufficiently homogeneous and has excessively large contrasts. Solutions to improve the homogeneity of the light from these lamps have already been realized. Treatments of the front surface of the glass sheet, such as frosting by sand-blasting or hot-patterning the surface of the glass, or else an opaline coloration through the thickness of the glass, have been proposed, among other suggestions, but are not sufficient and often too expensive. A satisfactory solution from the standpoint of homogeneity consists in covering the front face of the glass sheet with a plastic such as a frosted polycarbonate or acrylic polymer. However, this solution has the drawback of requiring several layers of plastic coating which result in an overall thickness of at least 5 mm. Such a coating thickness added to the other components making up the screen results in a considerable increase in the overall thickness of the lamp. This goes counter to the current trend towards reducing the overall size of screens in terms of thickness. The increase in the thickness also causes a reduction in the luminance of the lamp. Another drawback of such a lamp is that the plastic coating does not withstand the high temperatures required for its production, especially when carrying out the steps of depositing the electrodes and of sealing the periphery of the glass sheets.
The inventors were thus tasked with finding a means resulting in homogeneity of the light emitted by a flat lamp which is at least equivalent to the abovementioned solution but does not have its drawbacks, especially in terms of overall size and loss of luminance.